\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
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\begin{document}
\begin{equation*} d(\vec{x},\vec{y})=
\sqrt{\sum_{i=1}^p  {(x_i - y_i)^2 \over \sigma_i^2}},
\end{equation*}
\end{document}
